This invention relates to a tandem seal for sealing a shaft by means of two mechanical seals arranged in a tandem manner in an axial direction, which is used to seal a vaporizable liquid, particularly a liquefied gas, as an object to be sealed.
A tandem seal is a shaft sealing device in which two mechanical seals are arranged in a tandem manner in an axial direction. The tandem seal is preferably used with a liquefied gas, for instance, as a sealed object. In the tandem seal, even if large leakage occurs from a mechanical seal at a side of a primary mechanical seal, namely a side of the sealed liquid, a secondary mechanical seal, namely a side of an atmosphere, will be backup. When providing a collection mechanism in a center room between both mechanical seals for collecting the sealed liquid leaked from the primary mechanical seal to the center room, leakage to the side of the atmosphere can be securely prevented.
In the tandem seal, a sealed slide surface of the secondary mechanical seal, which comprises sealing elements at a rotational side and sealing elements at a stationary side, needs to be protected by liquid lubrication. Thereby a center liquid for lubrication is interposed in the center room that is arranged between both mechanical seals. Further, in order to eliminate slide heat generated at a sealed slide surface of the secondary mechanical seal, generally the center liquid is circulated via the outer cooler in a system.
FIG. 3 is a typical prior art showing a tandem seal used as a means for sealing the rotating shaft of the pump for a liquefied gas. Reference numeral 100 designates a rotating shaft of a pump. Numeral 110 designates a housing of a shaft sealing part so assembled that it surrounds an outer circumference of the rotating shaft 100 by a plurality of housing members 111xcx9c113 at a shaft sealing position. A shaft center of the rotating shaft 100 and the housing 110 extend almost horizontally. The left side in the figure means the inside of a machine. In the machine, a sealed liquid SL (a liquefied gas) exists in a pump room. The right side in the figure means the outside of the machine, namely, the side of the atmosphere.
In a space around the shaft between the housing 110 of the shaft sealing part and the rotating shaft 100, mechanical seals 120, 130 are provided in the axial direction. The seal 120 is a primary mechanical seal, which is provided to the inside of the machine. The seal 130 is a secondary mechanical seal, which is provided to the outside of the machine.
The primary mechanical seal 120 comprises a non-rotating fixed ring 121 and a rotating ring 123. The fixed ring 121 is supported via an O-ring 122 on the inner circumferential part of the housing member 112 of the housing 110. The rotating ring 123 rotates together with the rotating shaft 100. The rotating ring 123 is provided in a state that it is movable in the axial direction and is sealed with a sleeve 101 of the rotating shaft 100 via a retainer 124 and an O-ring 125. Further, the rotating ring 123 is closely contacted with the fixed ring 121 by an enforcement of springs 126 in the axial direction, which functions seal ability of the shaft on a sealed slide surface 120S.
Basically, the secondary mechanical seal 130 has the similar structure of the primary mechanical seal 120. That is, at the inner circumferential part of the housing member 113 of the shaft sealing part 110, it comprises a non-rotating fixed ring 131 and a rotating ring 133. The fixed ring 131 is supported via an O-ring 132. The rotating ring 133 rotates together with the rotating shaft 100. The rotating ring 133 is provided in a state that it is sealed and is movable in the axial direction via a pumping ring 134 and an O-ring 135 on the sleeve 101 of the rotating shaft 100. Further, the rotating ring 133 is closely contacted with the fixed ring 131 by enforcement in the axial direction of springs 136. This functions seal ability of the shaft on a sealed slide surface 130S.
The primary mechanical seal 120 is provided for the liquefied gas as the sealed object in the pump room in the machine. One part of the sealed liquid SL is sent to an outer circumferential room space A of the primary mechanical seal 120. Then it flows back to the pump room via a flushing discharge hole FOUT. Here the flushing discharge hole FOUT opens at the housing member 112 of the shaft sealing part housing 110. This functions as flushing. Thereby the sealed slide surface 120S of the primary mechanical seal 120 is lubricated and cooled.
Further, a center liquid CL lies in a center room B from the inner circumferential space of the sealed slide surface 120S of the primary mechanical seal 120 to the outer circumferential room of the sealed slide surface 130S of the secondary mechanical seal 130. The center liquid CL is flown back through a reservoir tank (not shown in the figure) installed outside, by axial flow pump action of the helical groove 134a of the pumping ring 134, via a circulation injection hole CLIN and a circulation discharge hole CLOUT. The pumping ring 134 is provided on the secondary mechanical seal 130. The circulation injection hole CLIN is opened at the housing member 112 of the shaft sealing part housing 110. The circulation discharge hole CLOUT is opened at the housing member 113. This circulation lubricates and cools the sealed slide surface 130S of the secondary mechanical seal 130. Therefore, the center liquid CL is usually filled in the flow line containing the center room B, the reservoir tank, the circulation injection hole CLIN, and the circulation discharge hole CLOUT.
In the tandem seal described above according to the prior art, the pumping ring 134 circulating the center liquid CL is arranged in the narrow center room B of the inner circumference of the shaft sealing part housing 110. Thus, there is a difficulty to raise pumping efficiency due to the size or limitation of the installing space of the sealing elements.
Further, the sealed liquid SL of the liquefied gas in the machine is pressed to a certain pressure, while the pressure of the center liquid CL in the center room B is low. Then, the sealed liquid SL leaked from the primary mechanical seal 120 to the center room B vaporizes in the center room B. When the bubble thereof in the center liquid CL increases, pumping efficiency by the pumping ring 134 decreases extremely. Moreover, near the rotating pumping ring 134, by the cavitation phenomena generated in the center liquid CL, the pumping ring 134 vibrates, and then this vibration is transmitted to the rotating ring 133 of the secondary mechanical seal 130. Thus, it has bad influence for sealing ability of the secondary mechanical seal 130.
Moreover, the bubble mixed in the center liquid CL by vaporization of the sealed liquid SL leaked from the primary mechanical seal 120 to the center room B sometimes forms gas zone as a result of gathering at the inner circumferential side of the center room B. This is because the center liquid CL in the center room B circulates and flows in the same direction as the rotation of the rotating shaft 100. This is caused by contact with the sealing elements at the rotational side of the secondary mechanical seal 130. Thus, effect of centrifugal separation by specific gravity occurs in the center liquid CL and the bubble of the leaked gas. Accordingly, the sealed slide surface 130S of the secondary mechanical seal 130 becomes a dry-slide state, namely a slide state without liquid lubrication by the gas zone. This will cause unusual wear or damage by overheats and will rapidly lower sealing function.
The object of the invention is to eliminate bad affects or influence for sealing function caused from a vaporized gas of the sealed liquid that is leaked from a primary mechanical seal into an intermediate chamber or center room formed between the primary mechanical seal and a secondary mechanical seal.
In a tandem seal according to the present invention, a primary mechanical seal and a secondary mechanical seal are arranged tandem in an axial direction around a rotating shaft in a space between the rotating shaft and a housing which constitutes a shaft sealing part, wherein the secondary mechanical seal is arranged outside of the primary mechanical seal in a machine, and wherein the housing of the shaft sealing part surrounds an outer circumference of the rotating shaft, wherein an intermediate or center liquid is sealed into a center room or intermediate chamber between both mechanical seals, wherein the center or intermediate liquid has an amount less than its capacity and enough to be stirred or agitated by a rotating part of the secondary mechanical seal, and wherein a gas vent line is provided in the housing of the shaft sealing part, wherein the gas vent line faces and opens to the center room. According to this invention, the center liquid unfilled in the capacity of the center room is stirred and scattered by a rotational side element of the secondary mechanical seal so as to lubricate the sealed slide surface. In comparison with the center liquid completely filled in the center room, heat generation due to the friction loss between the center liquid and the rotational side element of the secondary mechanical seal (that is heat generation by agitating) decreases. Therefore, the prior circulating and cooling mechanism for circulating and cooling of the center liquid is removed. The gas vaporized from the sealed liquid leaked from the primary mechanical seal in to the center room is naturally discharged from the gas vent line.
Further, in a tandem seal according to the invention , a baffle plate is provided with the housing of the shaft sealing part, wherein the baffle plate intersects a circumferential direction of the center room. Therefore, the center liquid, which is sealed in the center room by the amount unfilled in the capacity, collides with the baffle plate when it revolves and flows in the same direction as the rotation of the rotating shaft by contact with the elements at the rotating side. This leads to effective stirring and successful scattering, and prevents formation of gas zone in the center room.
Further, in a tandem seal according to the invention , a dividing plate, that is located between both mechanical seals, is provided in the housing of the shaft sealing part, wherein the dividing plate has an inner circumferential edge that opposes to the rotating shaft with a small space and divides roughly in the axial direction with the center room. According to the constitution, when generating large amount of leakage from the primary mechanical seal to the center room, it can prevent that the gas vaporized rapidly goes to the side of the secondary mechanical seal.
Further, in a tandem seal according to the invention, a coolant jacket is provided in the housing of the shaft sealing part, wherein a coolant from outside is flowed in the coolant jacket. The center liquid sealed in the center room may be cooled by the heat of vaporization (latent heat) of the leaked liquid on vaporizing from the primary mechanical seal. According to the constitution, cooling effect of the sealed slide surface of the second mechanical seal is raised, since the center liquid is certainly cooled by heat exchange of the coolant in the coolant jacket.